The invention relates generally to dye transfer printers such as thermal printers, and in particular to the problem of crease or wrinkle formation in successive dye transfer areas of a dye donor web. Crease formation in the dye transfer area can result in an undesirable line artifact being printed on a dye receiver.
A typical multi-color dye donor web that is used in a thermal printer is substantially thin and has a repeating series of three different rectangular-shaped color sections or patches such as a yellow color section, a magenta color section and a cyan color section. Also, there may be a transparent colorless laminating section immediately after the cyan color section.
Each color section of the dye donor web consists of a dye transfer area that is used for dye transfer printing and a pair of opposite longitudinal edge areas alongside the dye transfer area which are not used for printing. The dye transfer area is 152 mm wide and the two edge areas are each 5.5 mm wide, so that the total web width is 163 mm.
To make a multi-color image print using a thermal printer, a motorized donor take-up spool pulls the dye donor web from a donor supply spool in order to successively draw an unused single series of yellow, magenta and cyan color sections over a stationary bead of selectively heated resistive elements on a thermal print head between the two spools. Respective color dyes within the yellow, magenta and cyan color sections are successively heat-transferred, via the bead of selectively heated resistive elements, in superimposed relation onto a dye receiver such as a paper or transparency sheet or roll, to form the color image print. The bead of resistive elements extends across the entire width of a color section, i.e. across its dye transfer area and the two edge areas alongside the transfer area. However, only those resistive elements that contact the dye transfer area are selectively heated. Those resistive elements that contact the two edge areas are not heated. In other words, the dye transfer is effected from the dye transfer area to the receiver medium, but not from the two edge areas to the receiver medium.
As each color section, including its dye transfer area and the two edge areas alongside the transfer area, is drawn over the bead of selectively heated resistive elements, the color section is subjected to a longitudinal tension particularly by a pulling force of the motorized donor take-up spool. Since the dye transfer area is heated by the resistive elements, but the two edge areas alongside the transfer area are not, the transfer area is significantly weakened and vulnerable to stretching as compared to the two edge areas. Consequently, the longitudinal tension will stretch the dye transfer area relative to the two edge areas. This stretching causes the dye transfer area to become thinner than the non-stretched edge areas, which in turn causes some creases or wrinkles to develop in the transfer area, mostly in those regions of the transfer area that are close to the two edge areas. The creases or wrinkles occur mostly in the regions of the dye transfer area that are close to the two edge areas because of the sharp, i.e. abrupt, transition between the weakened transfer area and the stronger edge areas.
As the dye donor web is pulled by the motorized donor take-up spool over the bead of selectively heated resistive elements, the creases or wrinkles tend to spread from a trailing or rear end portion of a used dye transfer area at least to a leading or front end portion of the next dye transfer area to be used. A problem that can result is that the creases or wrinkles in the leading or front end portion of the next dye transfer area to be used will cause undesirable line artifacts to be printed on a leading or front end portion of the dye receiver, when the dye transfer occurs at the creases in the leading end portion of the next dye transfer area to be used. The line artifacts printed on the dye receiver are relatively short, but quite visible.
The question presented therefore is how to solve the problem of the creases or wrinkles being created in an unused transfer area so that no line artifacts are printed on the dye receiver during the dye transfer.
The Cross-Referenced Application
The cross-referenced application discloses a thermal printer capable of preventing crease formation in a dye transfer area of a dye donor web that can cause line artifacts to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver. The thermal printer, as disclosed, includes:
a thermal print head for heating the dye transfer area of the dye donor web sufficiently to effect a dye transfer from the dye transfer area to the dye receiver, but not heating opposite edge areas of the dye donor web alongside the dye transfer area sufficiently to effect a dye transfer from the edge areas to the dye receiver, so that the dye transfer area is vulnerable to being stretched relative to the two edge areas;
a web take-up roller that takes up the dye donor web, and that exerts a pulling force on the dye transfer area and two edge areas at the print head which is sufficient to stretch the dye transfer area relative to the two edge areas to possibly form some creases at least in respective regions adjacent the two edge areas; and
a crease-preventing platen roller that holds the dye transfer area and two edge areas against the print head during the dye transfer from the dye transfer area to the dye receiver, and which is adapted to mechanically cause the two edge areas to be stretched substantially the same as the dye transfer area when the dye transfer area and two edge areas are subjected to the pulling force of the web take-up roller, whereby crease formation that causes line artifacts to be printed on the dye receiver is at least substantially prevented.
In a preferred embodiment, the platen roller has a rotatable main portion and opposite end portions with respective lengths to hold the dye transfer area and the two edge areas alongside the dye transfer area against the thermal print head. The roller end portions have a diameter and a compliance that are greater than the diameter and compliance of the roller main portion, so that the roller end portions apply a pressure against the two edge areas that is greater than a pressure the roller main portion applies against the dye transfer area. This pressure difference causes the two edge areas to be stretched substantially the same as the dye transfer area when the edge areas and the dye transfer area are subjected to the pulling force of the web take-up roller.
A thermal printer capable of preventing crease formation in a dye transfer area of a dye donor web that can cause line artifacts to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver, said printer comprising:
a thermal print head for heating the dye transfer area of the dye donor web sufficiently to effect a dye transfer from the dye transfer area to the dye receiver, but not heating two opposite edge areas of the dye donor web alongside the dye transfer area sufficiently to effect a dye transfer from the two edge areas to the dye receiver, so that the dye transfer area is vulnerable to being stretched relative to the two edge areas;
a web take-up that takes up the dye donor web, and that exerts a pulling force on the dye transfer area and two edge areas at the print head which is sufficient to stretch the dye transfer area relative to the two edge areas to possibly form some creases in at least respective regions adjacent the two edge areas; and
a crease-preventing platen roller having a rotatable main portion and a coaxial pair of opposite end portions each rotatable independently of the roller main portion, with respective lengths to hold the dye transfer area and two edge areas against the print head, and with a diameter that is greater at the roller end portions than at the roller main portion, so that the roller end portions apply a pressure against the two edge areas that is greater than a pressure the roller main portion applies against the dye transfer area, whereby the pressure difference causes the two edge areas to be stretched substantially the same as the dye transfer area when the dye transfer area and two edge areas are subjected to the pulling force of the web take-up roller, so that crease formation which can cause line artifacts is at least substantially prevented.
In a preferred embodiment, the roller end portions rotatable independently of the roller main portion each have a single diameter periphery which applies pressure against the two edge areas at the print head.
In contrast, in an alternate embodiment, the roller end portions rotatable independently of the roller main portion each have dual diameter peripheries. In other words, there is a uniform diameter periphery for applying pressure against the two edge areas at the print head and a varying diameter periphery between the uniform diameter periphery and the roller main portion which decreasingly tapers from the diameter of the uniform diameter periphery to the diameter of the roller main portion.